A superconducting hot-electron bolometer has been built and tested as a prototype of high-sensitivity, rapid-response detectors of submillimeter-wavelength radiation. There are diverse potential applications for such detectors, a few examples being submillimeter spectroscopy for scientific research; detection of leaking gases; detection of explosive, chemical, and biological weapons; and medical imaging.

A Thin Nb Bridge having Tc = 6 K lies between thicker Nb contacts having Tc = 8.6 K that, in turn, are connected to an antenna that couples submillimeter- wavelength radiation into the device.
This detector is a superconducting-transition-edge device. Like other such devices, it includes a superconducting bridge that has a low heat capacity and is maintained at a critical temperature (Tc) at the lower end of its superconductingtransition temperature range. Incident photons cause transient increases in electron temperature through the superconducting- transition range, thereby yielding measurable increases in electrical resistance. In this case, Tc = 6 K, which is approximately the upper limit of the operating-temperature range of silicon-based bolometers heretofore used routinely in many laboratories. However, whereas the response speed of a typical silicon-based laboratory bolometer is characterized by a frequency of the order of a kilohertz, the response speed of the present device is much higher — characterized by a frequency of the order of 100 MHz.

For this or any bolometer, a useful figure of merit that one seeks to minimize is (NEP)τ1/2, where NEP denotes the noise-equivalent power (NEP) and τ the response time. This figure of merit depends primarily on the heat capacity and, for a given heat capacity, is approximately invariant. As a consequence of this approximate invariance, in designing a device having a given heat capacity to be more sensitive (to have lower NEP), one must accept longer response time (slower response) or, conversely, in designing it to respond faster, one must accept lower sensitivity. Hence, further, in order to increase both the speed of response and the sensitivity, one must make the device very small in order to make its heat capacity very small; this is the approach followed in developing the present device.

In the present device, the superconducting bridge having the Tc of 6 K is a thin film of niobium on a silicon substrate (see figure). This film is ≈1 μm wide, ≈1 μm long, and between 10 and 25 nm thick. A detector so small could lose some sensitivity if thermal energy were allowed to diffuse rapidly from the bridge into the contacts at the ends of the bridge. To minimize such diffusion, the contacts at the ends of the bridge are made from a 150-nm-thick niobium film that has a higher Tc(8.6 K). The interfaces between the bridge and the contacts constitute an energy barrier of sorts where Andreev reflection occurs. As a result, the sensitivity of the device depends primarily on thermal coupling between electrons and the crystal lattice in the Nb bridge. For this device, (NEP) = 2 × 10–14 W/Hz1/2 and the response time is about 0.5 ns.

In order to obtain high quantum efficiency, a planar spiral gold antenna is connected to the niobium contacts. The antenna enables detection of radiation throughout the frequency range from about 100 GHz to several terahertz. In operation, radiation is incident from the underside of the silicon substrate, and an antireflection-coated silicon lens (not shown in the figure) glued to the underside of the substrate focuses the radiation on the bridge (this arrangement is appropriate because silicon is transparent at submillimeter wavelengths).

This work was done by Boris Karasik, William McGrath, and Henry Leduc of Caltech for NASA’s Jet Propulsion Laboratory. NPO-43619

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Superconducting Hot-Electron Submillimeter-Wave Detector

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